Abstract

Long-distance quantum communication networks require appropriate interfaces between matter qubit-based nodes and low-loss photonic quantum channels. We implement a downconversion quantum interface, where the single photons emitted from a semiconductor quantum dot at 910 nm are downconverted to 1560 nm using a fiber-coupled periodically poled lithium niobate waveguide and a 2.2-μm pulsed pump laser. The single-photon character of the quantum dot emission is preserved during the downconversion process: we measure a cross-correlation g(2)(τ = 0) = 0.17 using resonant excitation of the quantum dot. We show that the downconversion interface is fully compatible with coherent optical control of the quantum dot electron spin through the observation of Rabi oscillations in the downconverted photon counts. These results represent a critical step towards a long-distance hybrid quantum network in which subsystems operating at different wavelengths are connected through quantum frequency conversion devices and 1.5-μm quantum channels.

Figures (4)

(a)–(d), Schematic diagrams of the experiment. The quantum dot sample was placed in a magnetic field (Voigt geometry) and was resonantly excited on the |↓〉–|↑↓, ⇓〉 transition (with Rabi frequency Ωex as in (c) and [7]) with the 910-nm excitation laser. The spin state was controlled with the 911-nm rotation laser (Rabi frequency Ωrot), which was synchronized to the excitation laser. A portion of the rotation laser power was picked off and combined with a strong 1565-nm laser to produce picosecond-pulsed 2.2-μm radiation used as the pump for the downconversion process. Single photons emitted via decay from |↑↓, ⇓〉 to |↑〉 were collected and downconverted in the PPLN waveguide with poling period ΛG = 21.9 μm. The resulting 1560-nm downconverted photons were detected on a superconducting nanowire single-photon detector (SNSPD).

Temporal characterization of the downconversion process: (a) DFG cross-correlation for a time delay between 2.2-μm pump pulses and 3-ps rotation laser pulses in the down-conversion waveguide, showing an approximately 10-ps-wide conversion time window, with good agreement to a numerical simulation. (b), Inset: SNSPD photon-count histogram for an integration time of 300 s showing converted QD single photons (red) and noise when the QD excitation is blocked (black), demonstrating very low leakage. Main: as the time delay between the excitation and pump pulse is varied, we trace out the spontaneous emission decay curve of the QD; the black dashed curve is an exponential decay with a 600-ps time constant, matching the value observed directly at 910 nm. For each time delay, we integrated for 300 s.

Single-photon intensity auto- and cross-correlations, showing strongly antibunched photon statistics. (a), Measured g(2)(τ) of the QD emission at 910 nm using two Si APDs and quasi-resonant excitation of the QD, showing a g(2)(0) = 0.13. Each bar represents an integral over a 5-ns time window around each peak. (b), Normalized cross correlation between nonconverted photons before the waveguide and downconverted photons at 1560 nm, showing g(2)(0) = 0.17. No background subtraction has been used. Error bars are one standard deviation assuming Poisson counting statistics.

(a), Rabi oscillations, observed as oscillations in the count rate at 910 (blue dots) and 1560 nm (green squares) as the excitation laser power is varied. (b) shows Rabi oscillations as the QD electron spin state is coherently rotated. In both plots, the solid blue curves (fit to the 910-nm data) are shown to guide the eye, and the vertical scales of both plots are adjusted to show the agreement between the count rate oscillations at 910 and 1560 nm. The data for the 1560-nm photons were integrated for 300 s per point.